The present invention generally relates to polishing a surface of a workpiece. More particularly, the invention relates to improved methods and apparatus for distributing fluids, for example slurry, to the surface of a polishing pad during chemical mechanical polishing.
Chemical mechanical polishing or planarizing a surface of an object may be desirable for several reasons. For example, chemical mechanical polishing is often used in the formation of microelectronic devices to provide a substantially smooth, planar surface suitable for subsequent fabrication processes such as photoresist coating and pattern definition. Chemical mechanical polishing may also be used to form microelectronic features. For example, a conductive feature such as a metal line or a conductive plug may be formed on a surface of a wafer by forming trenches and vias on the wafer surface, depositing conductive material over the wafer surface and into the trenches and vias, and removing the conductive material on the surface of the wafer using chemical mechanical polishing, leaving the vias and trenches filled with the conductive material.
A typical chemical mechanical polishing apparatus suitable for planarizing the semiconductor surface generally includes a wafer carrier configured to support, guide, and apply pressure to a wafer during the polishing process; a polishing compound such as a slurry containing abrasive particles and chemicals to assist removal of material from the surface of the wafer; and a polishing surface such as a polishing pad. In addition, the polishing apparatus may include an integrated wafer cleaning system and/or an automated load and unload station to facilitate automatic processing of the wafers.
A wafer surface is generally polished by moving the surface of the wafer to be polished relative to the polishing surface in the presence of the polishing compound. In particular, the wafer is placed in the carrier such that the surface to be polished is placed in contact with the polishing surface and the polishing surface and the wafer are moved relative to each other while slurry is supplied to the polishing surface.
The distribution of slurry over the polishing surface has been shown to be a critical factor in the chemical mechanical polishing process. The material removal rate across the surface of the wafer is generally related to the amount of slurry received by the polishing surface. Areas on the polishing surface having additional slurry will typically polish the wafer faster than areas on the polishing surface having less slurry. While the material removal rate may be fine tuned by intentionally adjusting the slurry distribution across the polishing surface, it is desirable to have a substantially uniform slurry distribution across the polishing surface.
One approach to distributing slurry across a polishing surface involves depositing the slurry from above in the middle of the polishing surface. Polishing surfaces typically move, for example, in a rotational, orbital or linear motion. The motion, in addition to removing material from the front surface of the wafer, helps to distribute the slurry across the polishing surface. However, this approach leads to a concentration of slurry in the middle of the polishing surface with the concentration of slurry declining in relation to its distance from the middle of the polishing surface.
Another approach to distributing slurry across a polishing surface involves pumping slurry from a cavity below the polishing surface through apertures in a platen and polishing surface to the polishing surface. However, the motions previously mentioned cause the slurry to concentrate along the periphery of the cavity and therefore, when forced to the polishing surface, the slurry is concentrated along the periphery of the polishing surface. As a partial correction for this problem, a cut o-ring has been spirally inserted into the cavity to reduce the concentration of slurry at the periphery of the polishing pad. However, the optimum shape of the cut spiral o-ring is difficult to determine and the optimum shape changes with different slurry delivery rates, speed of motions and types of slurry.
Another problem with using the cavity to distribute the slurry is the time it takes to change from a first slurry reaching the surface of the polishing pad to a second slurry reaching the surface of the polishing pad. Applicant has noticed the delay is caused by the cavity having a volume filled with the first slurry that must be completely replaced by the second slurry. The Applicant has also noticed the problem is compounded by parts of the cavity having no real flow direction resulting in a turbulent fluid motion. The turbulent fluid motion results in a mixing of the slurry and an additional time period when both slurries are delivered to the polishing surface further lengthening the time for a complete slurry change over.
What is needed is a method and apparatus for uniformly delivering a fluid to a polishing surface without being unduly affected by slurry delivery rates, speed of motions or types of slurry. The method and apparatus preferably allow a change in slurry to be quickly accomplished.
The present invention provides improved methods and apparatus for chemical mechanical polishing of a surface of a workpiece that overcome many of the shortcomings of the prior art. While the ways in which the present invention addresses the drawbacks of the now-known techniques for chemical mechanical polishing will be described in greater detail hereinbelow, in general, in accordance with various aspects of the present invention, the invention provides an improved method and apparatus for controlling the distribution of a fluid across a polishing surface.
The invention is a fluid delivery system for delivering a fluid to a polishing surface for a chemical mechanical polishing tool. The invention includes a platen, manifold and slurry delivery conduit. The platen supports the polishing surface and has a plurality of conduits for allowing a fluid to pass through the conduits in the platen and, preferably, through corresponding conduits in the polishing surface. This allows the fluid to reach the working area of the polishing surface. The platen may comprise several layers for performing additional functions not directly related to fluid distribution to the polishing surface.
The manifold controls the fluid distribution to the conduits to allow the fluid to pass through the platen and polishing pad. The manifold uses a plurality of channels in controlling the fluid distribution to the conduits of the platen. The channels may be formed in the manifold in a variety of ways, including, for example, by removing material from a monolithic manifold by machining or etching.
In one embodiment of the invention, the volume of all the channels is less than a third of the volume of the whole manifold. Reducing the volume of the channels reduces the time for a fluid change over. In another embodiment of the invention the channel cross-sectional area at substantially every point in the channels is greater than, preferably between by 1.5 and 2 times, the combined cross-sectional area of all the conduits being serviced by the channel. This causes the conduits in the platen to be the most restrictive feature in the fluid flow path resulting in a uniform backpressure in the manifold and therefore resulting in a uniform velocity of fluid flow. The cross-section of the channels may be incrementally changed or smoothly tapered.
The slurry delivery conduit communicates fluid from a fluid source to the channels in the manifold. The slurry delivery conduit is preferably in fluid communication with a central area of the manifold that feeds the plurality of channels. The slurry delivery conduit may be in fluid communication with a plurality of fluid sources so that a plurality of different fluids, preferably one at a time, may be communicated to the channels in the manifold as desired.
In operation, a fluid may be distributed across a polishing surface by pumping a fluid from a fluid source to a central area connected to a plurality of channels in a manifold. The fluid is communicated through the plurality of channels in the manifold to a plurality of conduits in a platen. Incrementally changing or tapering the channels as previously described results in a substantially uniform velocity of the fluid throughout the channels. The fluid travels through the conduits in the platen and through the polishing surface, preferably through corresponding conduits in the polishing surface, to reach a working area of the polishing surface.